Many cancers are characterized by disruptions in cellular signaling pathways that lead to aberrant control of cellular processes, or to uncontrolled growth and proliferation of cells. These disruptions are often caused by changes in the phosphorylation state, and thus the activity of, particular signaling proteins. Among these cancers is non-small cell lung carcinoma (NSCLC). NSCLC is the leading cause of cancer death in the United States, and accounts for about 87% of all lung cancers. There are about 151,000 new cases of NSCLC in the United States annually, and it is estimated that over 120,000 patients will die annually from the disease in the United States alone. See “Cancer Facts and Figures. 2003,” American Cancer Society. NSCLC, which comprises three distinct subtypes, is often only detected after it has metastasized, and thus the mortality rate is 75% within two years of diagnosis.
NSCLC, like most cancers, involves defects in signal transduction pathways. Receptor tyrosine kinases (RTKs) play a pivotal role in these signaling pathways, transmitting extracellular molecular signals into the cytoplasm and/or nucleus of a cell. Among such RTKs are the receptors for polypeptide growth factors such as epidermal growth factor (EGF), insulin, platelet-derived growth factor (PDGF), neurotrophins (i.e., NGF), and fibroblast growth factor (FGF). Phosphorylation of such RTKs activates their cytoplasmic domain kinase function, which in turns activates downstream signaling molecules. Thus, RTKs are key mediators of cellular signaling as well as oncogenesis resulting from over-expression and activation of such RTKs and their substrates. Due to their pivotal role in normal and aberrant signaling, RTKs are the subject of increasing focus as potential drug targets for the treatment of certain types of cancer. For example, Herceptin®, an inhibitor of HER2/neu, is currently an approved therapy for a certain subset of breast cancer. Iressa™ (ZD1839) and Tarceva™ (OSI-774), both small-molecule inhibitors of EGFR, have been approved for the treatment of NSCLC.
Platelet-derived growth factor (PDGF) and its receptors (PDGFRs) are a family of RTKs that play an important role in the regulation of normal cell growth and differentiation. PDGFRs are involved in a variety of pathological processes, including atherosclerosis, neoplasia, tissue repair, and inflammation (see, e.g. Ross et al., Cell 46: 155-169 (1986); Ross et al., Adv. Exp Med. Biol. 234: 9-21 (1988)). PDGFRs, which consist of two isoforms (alpha (α) and beta (β)), are membrane protein-tyrosine kinases that, upon binding to PDGF, become activated and, via recruitment of SH2 domain-containing effector molecules, initiate distinct or overlapping signaling cascades that coordinate cellular responses.
Expression of a constitutively active PDGFR leads to cellular transformation (see Gazit et al., Cell 39: 89-97 (1984)) and suggests that, in normal cells, PDGFR activity must be tightly regulated to oppose continuous activation of its downstream effectors. PDGFR beta, for example, is known to be over-expressed in a large number of tumors, and PDGF treatment causes transformation and malignant tumors in a variety of experimental systems (reviewed in Heldin et al., Physiol. Rev. 79(4): 1283-1316 (1999)). It has therefore been proposed that over-expression or constitutive activation of the PDGF receptors plays a role in the origin or tumorigenesis of certain cancer cells. It has been reported that PDGFR is activated by a fusion to the transcription factor TEL (see Ide et al., PNAS 99(22): 14404-14409 (2002)) in a subset of patients with chronic myelomonocytic leukemia (CML). PDGFR activation has also been implicated in growth of certain solid tumors, such as glioblastoma (see, e.g. Vassbotn et al., J. Cell. Physiol. 158: 381-389 (1994)).
Accordingly, inhibition of PDGFR and its downstream pathway has become an area of increasing focus for drug development. Specific inhibitors of PDGFR, such as the small-molecule drug Gleevec® (STI-571; Imatinib mesylate), have recently been developed and are in clinical trials for treatment of certain cancers, including prostate and ovarian cancers. It has been shown that Gleevec® induces durable responses in patients with chronic myelo-proliferative diseases associated with activation of PDGFR (see Apperley et al., N. Engl. J. Med 347(7): 481-7 (2002)). However, while PDGFR expression has been linked to the progression of a few cancers, such as CML and glioblastoma, this association has not been made in many other types of cancers. Similarly, although certain signaling defects underlying progression of NSCLC have been identified (including EGFR over-expression), the precise molecular mechanisms driving this disease are not completely understood.
One study reported the apparent expression of PDGFR alpha (α) in nearly 100% of human lung cancer tumors examined, and reported the growth inhibition of a lung cancer cell line, A549, by Gleevec®, an effect that was surmised to be mediated via PDGFR inhibition (see Zhang et al., Mol. Cancer 2(1): 1-10 (2003)). The report, however, was inconclusive since the antibody employed in the study was later shown (by the present inventors) to be non-specific, and cross-reacts with a variety of proteins other than PDGFRα; thus it is unclear which protein(s) was/were actually being detected in the Zhang study. Moreover, PDGFRα is not detectable in the A549 cell line employed in that study—which is consistent with the present inventors' inability to reproduce the growth inhibition of this cell line by Gleevec®—further evidencing that the observation reported in Zhang was either erroneous or was mediated by some mechanism other than expression and inhibition of PDGFRα.
Since the new generation of targeted therapeutics against RTKs like PDGFR and EGFR are highly specific, there is a continuing and imperative need to identify the particular tumors that are, in fact, being driven by the RTK being targeted by these drugs, since such tumors are most likely to respond to the inhibitor. It is now widely accepted that most types of cancer have distinct subsets of tumors, which are being driven by different signaling pathways. For example, two distinct subsets of breast cancer are known to exist, one driven by Her2/Neu signaling and the other by EGFR signaling, but only the former is responsive to the Her2-targeted therapeutic Herceptin®. It is likely that most types of cancer, including those in which an RTK has already been identified (and targeted) as a driver of the disease, will in fact have multiple subtypes being driven by other, presently unknown RTKs and pathways. Indeed, the modest response rates thus far observed in clinical trials of several highly specific targeted therapeutics (including those against EGFR and PDGFR) evidence that many of the cancers being treated may, in fact, comprise subgroups being driven by alternative RTKs and pathways that are not being adequately targeted.
Accordingly, there is a continuing and pressing need to identify the particular signaling molecules, including RTKs, whose expression and/or activation is, in fact, driving a certain type of cancer (or a subset of that cancer). Identification of such signaling molecules will enable the development of new and improved diagnostic and/or prognostic assays to help ensure a particular patient gets a targeted therapeutic most likely to be effective against their disease, as well as providing novel drug targets for treatment of these cancers. Some cancers, like NSCLC, are often not detected until after the disease has already metastasized, making prompt and effective treatment paramount. Therefore, the ability to identify subgroups of cancers that are being driven by presently-untargeted RTKs and signaling pathways would greatly assist in developing alternative and more beneficial therapeutic strategies, and to avoiding prescribing ineffective therapies to patients who are not likely to respond to them.